Epidemiologic and other evidence suggests that endocrine factors are important in the genesis or growth of malignant tumors of the mammary gland, and large numbers of women have been using estrogenic hormones premenopausally, for contraception, and postmenopausally, for replacement. Required is a means to monitor groups of women with breast cancer so as to be able to identify an excess in the number with a history of use of estrogens if such should occur during the next several years. Toward this end, we propose to identify residents of the city of Baltimore who are first found to have a malignant breast lesion during a five year interval, 1973-1977. These women, women who live in their neighborhood, and women who were in the same hospital for some other reason will be interviewed to obtain histories of use of estrogenic preparations and other related information. We expect to identify about 400 women with breast cancer each year and to interview perhaps 1900 women each year, including about 600 women with a breast mass that proved to be benign. We also contemplate creation of a registry of all female residents of Baltimore City first diagnosed with breast cancer during the years of this study.